It is possible to create Range objects that overlap in a document.
When overlapping ranges are used by multiple different Highlight objects, and when those highlights are styled using ::highlight pseudo-elements, this may lead to conflicting styles.
If two text ranges overlap and are both highlighted using the CSS Custom Highlight API, and if they're both styled using the color CSS property, the browser needs to decide which color should be used for styling the text in the overlapping part.
By default, all highlights have the same priority and the browser chooses the most recently registered highlight to style the overlapping parts.
The priority property of the Highlight interface is a Number used to change this default behavior and determine which highlight's styles should be used to resolve style conflicts in overlapping parts.
Note that all the styles of a highlight are applied and the browser only needs to resolve conflicts when the same CSS properties are used by multiple overlapping highlights. The highlight style conflict resolution also does not depend on the order in which the ::highlight pseudo-elements rules appear in the source, or whether or not CSS properties are marked as !important.
HTML
<p>Time is an illusion. Lunchtime doubly so.</p>
CSS
::highlight(highlight-2) {
color: blue;
}
::highlight(highlight-1) {
color: white;
background: orange;
}
JavaScript
const text = document.querySelector("p").firstChild;
const range1 = new Range();
range1.setStart(text, 5);
range1.setEnd(text, 25);
const range2 = new Range();
range2.setStart(text, 15);
range2.setEnd(text, 35);
const highlight1 = new Highlight(range1);
const highlight2 = new Highlight(range2);
CSS.highlights.set("highlight-1", highlight1);
CSS.highlights.set("highlight-2", highlight2);
Result
As seen below, by default, the part of the text node where the two registered highlights overlap is displayed in blue because highlight-2 is registered after highlight-1. The background color defined by highlight-1 includes the entire range1 range because it does not conflict with another background color.
HTML
<button id="prioritize-1" type="button">Prioritize 1</button>
<button id="prioritize-2" type="button">Prioritize 2</button>
<button id="reset" type="button">Reset</button>
<p>Time is an illusion. Lunchtime doubly so.</p>
CSS
::highlight(highlight-1) {
background-color: blue;
color: white;
}
::highlight(highlight-2) {
background-color: orange;
}
JavaScript
const text = document.querySelector("p").firstChild;
const range1 = new Range();
range1.setStart(text, 5);
range1.setEnd(text, 25);
const range2 = new Range();
range2.setStart(text, 15);
range2.setEnd(text, 35);
const highlight1 = new Highlight(range1);
const highlight2 = new Highlight(range2);
CSS.highlights.set("highlight-1", highlight1);
CSS.highlights.set("highlight-2", highlight2);
const prioritize1 = document.querySelector("#prioritize-1");
const prioritize2 = document.querySelector("#prioritize-2");
const reset = document.querySelector("#reset");
prioritize1.addEventListener("click", () => {
highlight1.priority = 1;
highlight2.priority = 0;
});
prioritize2.addEventListener("click", () => {
highlight1.priority = 0;
highlight2.priority = 1;
});
reset.addEventListener("click", () => {
highlight1.priority = 0;
highlight2.priority = 0;
});
Result
As seen below, by default, the part of the text node where the two registered highlights overlap is displayed in blue because highlight-2 is registered after highlight-1.